It is known to desalinate salt water by Capacitive Deionization (CDI) (also sometimes known as Electrostatic Deionization). The process basically consists in passing the salt water between a pair of electrodes, each of large surface area, between which a DC voltage is applied. Positive ions (e.g Na+ ions) migrate to the cathode, and negative ions (e.g Cl− ions) migrate to the anode. The adsorbed ions are then bound to the respective electrodes. From time to time, the stored ions are removed from the electrodes by an appropriate regeneration process.
Typically, in the conventional CDI cells, the electrodes are in the form of flat plates or sheets of e.g activated carbon. Salt water flows along the space between the plates, the ions being attracted to the appropriate electrode by electrostatic forces. Thus, the ions are adsorbed onto the respective electrodes from the passing water.
A conventional CDI-based treatment apparatus generally includes several of the cells, arranged in a stack of cells, and includes suitable structure for mounting the electrodes of the individual CDI cells, and for conveying the water into and through the spaces between the electrodes.
Ions are adsorbed into the porous material of the electrodes, and are retained and stored therein, whereby the effluent water from the CDI cell is less salty than the influent water.
For regeneration, usually the flow of salt water undergoing treatment is switched off, or re-routed, and a flow of regeneration water is now passed through the CDI cell. (In some cases, the regeneration water can be the same salt water.) Traditionally, the polarity of the cells is reversed, whereby the adsorbed ions are repelled from the electrodes, and enter the regeneration water. Typically, regeneration is carried out a few times per hour, and the regeneration process is typically completed in a few minutes. The treatment/regeneration cycle preferably should be automated.
The salt content of the effluent regeneration water is usually considerably (e.g ten times) higher than that of the salt water being desalinated. Where the salt water is drawn from the sea, the high-salt regen-water is simply discharged into the sea. If disposal in the sea is not available, further treatment of the concentrate stream might be required; however, the volume of the concentrate is typically only about five percent of the treated water stream.
Conventional CDI cells may or may not be provided with charge-barriers, which are ion-permeable membranes that are impervious to water, and placed over one or both of the electrodes. Charge barriers are aimed at preventing contamination of the electrode pore volume with the source water and to prevent re-adsorption of the ions during regeneration.